This invention relates to polycarbonate membranes containing substituted benzenes which interact with the polycarbonate polymer to enhance the gas separation performance of said membranes. The invention further relates to a process for fabricating said polycarbonate gas separation membranes containing substituted benzenes. The invention also further relates to a process of gas separation using said membranes.
The use of polymeric membranes for gas separation is well known. A wide variety of polymers has been used for gas separation membranes, including cellulose esters, polyamides, polyimides, polysulfones, and polyolefins. Gases successfully separated from gas mixtures by membranes include hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, water vapor, ammonia, and/or light hydrocarbons such as methane, ethane, ethylene, propane, propylene, butane, butylene, etc. An application of particular interest is membrane separation of oxygen and nitrogen from air. An enriched nitrogen stream obtained from air may be used for inert padding of flammable fluids or for food storage. An enriched oxygen stream obtained from air may be used for enhancing combustion or for increasing the efficiency of fermentation processes. Additional applications of interest include membrane separation of hydrogen from gas mixtures containing other gases such as nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons, which are formed in various hydrocracker, hydrotreater, and catalytic reformer processes used in the oil refinery industry.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the gas mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other component of the gas mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component or components and a stream which is depleted in the selectively permeating component or components. The stream which is depleted in the selectively permeating component or components is enriched in the relatively non-permeating component or components. A relatively non-permeating component passes through the membrane more slowly than at least one other component of the gas mixture. An appropriate membrane material is chosen for the gas mixture so that some degree of separation of the gas mixture can be achieved.
Polycarbonate membranes in particular have been discovered to possess good separation properties for gases, especially oxygen and nitrogen. Polycarbonate membranes possess good mechanical properties and are able to operate under extreme conditions of temperature and pressure. However, polycarbonate membranes sometimes possess lower gas flux or permeability or gas selectivity than desired for certain applications. A method of substantially increasing the gas flux or permeability and/or gas selectivity of such membranes without significantly decreasing the mechanical strength of such membranes is needed.